Fuel injectors for delivering predetermined quantities of fuel into a combustion space of an internal combustion engine or the like generally comprise a piston which is moveable within an internal bore of the injector in order to control the pressure of fuel contained within a control chamber of the injector. The control chamber is defined, in part, by a surface associated with a valve needle such that changes in fuel pressure within the chamber effect movement of the valve needle, thereby causing delivery of the fuel into the combustion space of the engine.
It is known to provide such fuel injectors with piezoelectric actuators for controlling movement of the piston within the bore. Such actuators generally consist of a piezoelectric body in the form of a multi-layer laminate or “stack” having layers of piezoelectric or piezoceramic material separated by metal or electrically conductive layers acting as electrodes.
The piezoelectric actuator is arranged to convert electrical energy into mechanical or kinetic energy such that an electrical voltage applied to the electrodes causes the piezoelectric material to expand or contract, in dependence on the magnitude and polarity of the voltage applied, thereby changing the effective length of the stack. The change in length of the stack effects movement of the control piston within the injector so as to open and close the valve needle.
It is known that the amount of compressive load applied to the piezoelectric stack can significantly influence actuator performance and durability. More specifically, the piezoelectric or piezoceramic material from which the piezoelectric stack is formed is capable of withstanding compressive stress but cannot withstand significant tensile stress. It is therefore advantageous to ensure that a compressive load on the stack is maintained throughout the operating cycle of the injector.
In known actuator arrangements, the compressive load applied to the piezoelectric stack is provided solely by the hydrostatic pressure of the fuel contained within an accumulator volume within which the stack is located. An actuator of this type is described in our co-pending, published European patent application, EP 0995901 A. However, during the operating cycle of the injector, it has been found that the hydrostatic pressure of the fuel can vary significantly, to the extent that the compressive load on the stack may drop below that needed for acceptable actuator durability.
Accordingly, it would be advantageous to have a system and method for maintaining a predetermined or minimum compressive load on the stack substantially throughout the operating cycle of the actuator.